How to MegaSquirt Your Subaru RX Turbo

Building a Harness Adaptor to use the stock wiring harness for full control of the stock fuel and ignition systems with the MegaSquirt-II

Photo by John Swain

Application: 1987 Subaru RX Turbo — Other surrounding year models are very likely the same, check ECU pinout and distributor part# to compare.

Level of control: Full Standalone EMS– Fuel and Ignition Control

As featured in SubieSport Magazine, our adventures of installing and dialing in a Megasquirt-II on Will Maham’s Scooby RX Turbo. The wiring portion of this car is really pretty straightforward, we had to play around with the ignition settings within the software a bit to get it all right and then spent a bit of time on the dyno but our results ended up looking pretty good I think considering we made no other changes and still made 10hp and 8lbs of torque at the wheels peak, and a healthy increase across the band– before turning up the boost… see below for full dyno results.

So without further ado here’s the wiring diagram for doing the exact same thing to your own Subaru RX Turbo–

This guide will allow you to build your own ‘harness adaptor’ to do a Plug-n-play style installation of the MegaSquirt-II ECU into your Subaru RX Turbo that will look something like this (before you wrap it up and make it pretty):

You can see the gray wire looped around on the larger connector from one pin to another, if you look up at the schematic above this is the wire labeled ’12v power source to Optical CAS in Dizzy’. You can also see that we used this 12v source to connect one end of a 1k 1/4watt resistor to, and connected the other end to a bit of wire running to pin 17 of that same large connector, also shown on the schematic above. This is the ‘TACH Pullup’ which ‘pulls’ the Optical CAS sensors output up to 12v so that the MegaSquirt can read it properly. It’s looking for a 12v square wave on the ignition input (TACH input–pin24 to the MS-II).

You can source your own wire to build this out if you’d like, or you can use one of my universal wire harnesses as a basis to provide the color coded (hi-temp automotive grade) wiring already attached to the MegaSquirt ECU connector, then you just need to dig up the Subaru ECU connector from a junkyard ECU and solder up the one side. For Will’s car I used some spare wire I had laying around the shop instead of one of my harnesses as I don’t mind soldering all the wires to the connector and I had to use that wire for something….

And the only bit of underhood wiring is: You need to mount an IAT sensor in your intake tube just before the throttle body and run the two wires into the stock MAF connector where you’ve unplugged the MAF. Here’s a picture of how to wire it into the MAF connector, the sensor isn’t polarized so which wire goes into which MAF connector hole isn’t important, just that you hook them into the connector as seen above. This will route the IAT sensor signal through your stock wiring harness, back to the ‘harness adaptor’ you built, and to the MegaSquirt-II.

Here is a link to my Sensors catalog where you can find the IAT sensor and a weld-on bung you can use to fit it to your intake tube.

TunerStudio Configuration

Note that your mileage may vary with this configuration info– the core of it should be pretty accurate for a similar vehicle, basically everything except the VE table and the Ignition Table. Those tables you’ll want to go over and dial in for your specific ride, these will likely be pretty close if you car’s mods are similar to Will’s. Pretty much stock motor with stock injectors, stock turbo, aftermarket downpipe and exhaust, A2W Intercooler, etc. Even if your car is an exact duplicate… take the time to make sure the map is right…. it’s your responsibility to tune it in– i’m trying to take care of some of the guesswork for you though ;). This is what worked on Will’s car:

One more note– we never went over 10-11psi even though these tables are setup for much more, keep that in mind, we’ve tuned nothing above 10-11 psi (about 170 kpa) so that info is a total guesstimate up above there…

Dyno Results

This first map compares the stock to stock, only change is the ECU and tuning of the Megasquirt-II EMS. The peak numbers tell a bit of the story, with a little over 10hp and 8lbs of torque at the wheels, but the real benefit is in the broad gains all across the powerband where’s there’s a 5 to as much as a 17hp gain around 5200rpm. And similarly a 5-15 ft-lb torque bump through much of the range… this made all the difference in the world on the autocross course, where peak peak power means very little (still nice to have gained there though), and broad torque means everything.

This second map also has the stock to stock comparison, but adds a 10-11psi run after we installed a MBC and turned it up a bit. Here we made an additional 13.6whp and 16.5 ft-lb of torque on top of what we made at 7psi, and you can see we worked on our tuning a bit more and got that nasty dip at 2500 tuned out as well. And you want to talk about torque increase across the band? Compared to stock we’re now looking at a 24whp and 15 ft-lb increase starting at 2900rpms when the turbo has finished spooling up, and staying pretty solid increasing to more than a 40 whp and 40 ft-lb increase just above 5000rpm and carrying most of that all the way to redline… Too bad Will couldn’t take it too the track this way– he had to drop it back to 7psi for his class, but it was fun on the street for a bit…